A weakness of some PABX systems lies in the distant voicemail feature, which is accessed by entering a password when the initial greeting is being played. A hacker can call a direct dial number with voicemail, and then try to use the default password or guess it, or then select the "call back" function, and enter a premium rate number for the call back. The PABX calls back the premium rate line, confirming the password for the hacker. To stop this form of hacking, it is important to turn off the call back feature on the PABX, or to use a strong password.

Mobile phone companies usually allow access to associated voicemail messages using a landline telephone. This requires the entry of a Personal Identification Number (PIN). Many mobile phone companies use a system that sets a well-known four digit default PIN that is rarely changed by the phone's owner, making it easy for a hacker who knows both the phone number and the service provider to access the voicemail messages associated with that service.[4] Even where the default PIN is not known, social engineering can be used to reset the voicemail PIN code to the default, by impersonating the owner of the phone with a call to a call centre.[5][6] Many people also use weak PINs that are easily guessable; to prevent subscribers from choosing PINs with weak password strength, some mobile phone companies now disallow the use of consecutive or repeat digits in voicemail PIN codes.[7]

During the mid-2000s, it was discovered that calls emanating from the handset registered against a voicemail account were put straight through to voicemail without needing to enter a PIN. A hacker could use caller ID spoofing to impersonate a victim's handset caller ID and thereby gain access to the associated voicemail without a PIN.[8][9][10]

Following controversies over phone hacking and criticism that was levelled at mobile service providers who allowed access to voicemail without a PIN, many mobile phone companies have strengthened the default security of their systems so that remote access to voicemail messages and other phone settings can no longer be achieved via a default PIN.[4] For example, AT&T announced in August 2011 that all new wireless subscribers would be required to enter a PIN when checking their voicemail, even when checking it from their own phones, while T-Mobile stated that it "recommends that you turn on your voice mail password for added security, but as always, the choice is yours."[11]

An analysis of user-selected PIN codes suggested that ten numbers represent 15% of all iPhone passcodes, with "1234" and "0000" being the most common, with years of birth and graduation also being common choices.[12] Even if a four-digit PIN is randomly selected, the key space is very small ({\displaystyle 10^{4}} or 10,000 possibilities), making PINs significantly easier to brute force than most passwords; someone with physical access to a handset secured with a PIN can therefore feasibly determine the PIN in a short time.[13]

Mobile phone microphones can be activated remotely by security agencies or telcos, without any need for physical access, as long as the battery has not been removed.[14][15][16][17][18][19] This "roving bug" feature has been used by law enforcement agencies and intelligence services to listen in on nearby conversations.[20]

Other techniques for phone hacking include tricking a mobile phone user into downloading malware which monitors activity on the phone. Bluesnarfing is an unauthorized access to a phone via Bluetooth.[6][21]

There are flaws in the implementation of the GSM encryption algorithm that allow passive interception.[22] The equipment needed is available to government agencies or can be built from freely available parts.[23]

In December 2011, German researcher Karsten Nohl revealed that it was possible to hack into mobile phone voice and text messages on many networks wiuth free decryption software available on the Internet. He blamed the mobile phone companies for relying on outdated encryption techniques in the 2G system, and said that the problem could be fixed very easily.[24]